Scattering of ultrasound by biological tissues is not yet fully understood. Our long term objective is to gain such an understanding with the aim of increasing the utility of ultrasonic diagnostic procedures. Recent investigations by various researchers have shown the importance of knowing the phase-cancellation errors in the masurement of scattered fields. One of our important objectives is to determine by theoretical analyses and/or computer simulation the extent of phase cancellation errors in various experiments on different types of tissues. Our other objectives will include using Monte Carlo computer simulation procedures to determine the importance of multiple scattering in tissues and to examine the techniques of Fourier imaging for directly obtaining the images of scattering centers.